linux-kernel - Re: [RFC PATCH v6 27/29] x86/mm/pti: Implement a TLB flush immediately after a switch to kernel CR3

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Message-ID: <aQIpXyX-z8ltB1i5@localhost.localdomain>
Date: Wed, 29 Oct 2025 15:49:03 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Valentin Schneider <vschneid@...hat.com>
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Subject: Re: [RFC PATCH v6 27/29] x86/mm/pti: Implement a TLB flush
 immediately after a switch to kernel CR3

Le Wed, Oct 29, 2025 at 03:13:59PM +0100, Valentin Schneider a écrit :
> On 29/10/25 11:31, Frederic Weisbecker wrote:
> > Le Wed, Oct 29, 2025 at 11:16:23AM +0100, Valentin Schneider a écrit :
> >> On 28/10/25 16:59, Frederic Weisbecker wrote:
> >> > Le Fri, Oct 10, 2025 at 05:38:37PM +0200, Valentin Schneider a écrit :
> >> >> @@ -171,8 +172,27 @@ For 32-bit we have the following conventions - kernel is built with
> >> >>      andq    $(~PTI_USER_PGTABLE_AND_PCID_MASK), \reg
> >> >>  .endm
> >> >>
> >> >> -.macro COALESCE_TLBI
> >> >> +.macro COALESCE_TLBI scratch_reg:req
> >> >>  #ifdef CONFIG_COALESCE_TLBI
> >> >> +	/* No point in doing this for housekeeping CPUs */
> >> >> +	movslq  PER_CPU_VAR(cpu_number), \scratch_reg
> >> >> +	bt	\scratch_reg, tick_nohz_full_mask(%rip)
> >> >> +	jnc	.Lend_tlbi_\@
> >> >
> >> > I assume it's not possible to have a static call/branch to
> >> > take care of all this ?
> >> >
> >>
> >> I think technically yes, but that would have to be a per-cpu patchable
> >> location, which would mean something like each CPU having its own copy of
> >> that text page... Unless there's some existing way to statically optimize
> >>
> >>   if (cpumask_test_cpu(smp_processor_id(), mask))
> >>
> >> where @mask is a boot-time constant (i.e. the nohz_full mask).
> >
> > Or just check housekeeping_overriden static key before everything. This one is
> > enabled only if either nohz_full, isolcpus or cpuset isolated partition (well,
> > it's on the way for the last one) are running, but those are all niche, which
> > means you spare 99.999% kernel usecases.
> >
> 
> Oh right, if NOHZ_FULL is actually in use.
> 
> Yeah that housekeeping key could do since, at least for the cmdline
> approach, it's set during start_kernel(). I need to have a think about the
> runtime cpuset case.

You can ignore the runtime thing and simply check the static key before reading
the housekeeping mask. For now nohz_full is only enabled by cmdline.

> Given we have ALTERNATIVE's in there I assume something like a
> boot-time-driven static key could do, but I haven't found out yet if and
> how that can be shoved in an ASM file.

Right, I thought I had seen static keys in ASM already but I can't find it
anymore. arch/x86/include/asm/jump_label.h is full of reusable magic
though.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs